Hematopoiesis is the process by which blood cells develop and differentiate from pluripotent stem cells in the bone marrow. This process involves a complex interplay of polypeptide growth factors (cytokines) acting via membrane-bound receptors on their target cells. Cytokine action results in cellular proliferation and differentiation, with a response to a particular cytokine often being lineage-specific and/or stage-specific. Development of a single cell type, such as a platelet, from a stem cell may require the coordinated action of a plurality of cytokines acting in the proper sequence.
It was hypothesized for many years that the production of platelets may be regulated by specific humoral factors. Early experiments had shown that plasma or urine of thrombocytopenic animals contains an activity that promotes megakaryocytic colony formation and increases the size of marrow megakaryocytes. This activity is referred to in the literature as "thrombopoietin" (recently reviewed by McDonald, Exp. Hematol. 16: 201-205, 1988 and McDonald, Am. J. Ped. Hematol. Oncol. 14: 8-21, 1992). The low concentration of this activity and the lack of suitable bioassays long hampered the purification and characterization of the protein. Thrombopoietin has now been produced using genetically engineered cultured cells. See, de Sauvage et al., Nature 369: 533-538, 1994; Lok et al., Nature 369: 565-568, 1994; Kaushansky et al., Nature 369: 568-571, 1994; and Bartley et al., Cell 77: 1117-1124, 1994.
Thrombopoietin has been shown to increase platelet numbers in normal (Lok et al., ibid.) and thrombocytopenic (Sprugel et al., Blood 84 (10 Suppl. 1): 242a, 1994) animals, and to stimulate production of erythrocytes (Kaushansky et al., J. Clin. Invest., in press). In vitro, TPO enhances survival and proliferation of CD34.sup.+ cells destined to become megakaryocytes (Papayannopoulou et al., Blood 84 (10 Suppl. 1): 324a, 1994).
Although the cloning and characterization of TPO now permits investigation of its clinical use in stimulating thrombopoiesis, thrombocytopenia and anemia remain as significant clinical problems, such as in connection with chemotherapy and radiation therapy of cancer patients. There remains a particular need for methods of stimulating platelet production in patients receiving bone marrow transplants and peripheral blood stem cell transplants, including autologous transplants. There also remains a need for stimulating erythrocyte production. The present invention provides therapeutic methods that address these needs, and provides other, related advantages.